Musk said "the most fun plane" he owned was an Aero L-39 Albatros, which Russian military pilots use as trainers for flying fighter jets.

Musk later decided that flying the jet was "crazy" because he had kids to raise and companies to run.

Elon Musk made his first millions from the sales of Zip2 and PayPal, then invested much of the money into founding SpaceX in 2002 and co-founding Tesla in 2003.

But in addition to launching two of the world's most famous startups, Musk also devoted some cash to his love of aviation. He purchased a Piper Meridian single-engine turboprop, a two-engine Cessna Citation CJ2 corporate jet, and a high-speed military jet called the Aero Vodochody L-39 Albatros.

The L-39 jet was designed and built during the Cold War in Czechoslovakia, which was a satellite state of the Soviet Union.

Engineers created the high-performance aircraft as a training vehicle for pilots before they climbed inside combat-grade fighter jets.

"Probably the most fun plane I have is a Russian fighter jet," Musk told Fortune magazine in 2003. "It has a Czech air frame, a Ukrainian engine, Russian avionics. It's what they used to train their fighter pilots on, so it's incredibly acrobatic. But your butt hurts if you fly in it for more than an hour. The seats are really hard."

'It was just like Top Gun'

Although a popular color scheme is Cold War-era camouflage, an alleged September 2006 photo of Musk's jet at an airport Reno, Nevada, suggests his had glossy black paint, red and chrome pinstripes down the side, and a "SpaceX" logo.

"You can do eight Gs in that plane," Musk told Fortune, referring to eight times the force of Earth's gravity on the planet's surface — enough acceleration to make a pilot black out without a proper flight suit.

Due to that risk, Musk says he only flew it up to about five Gs. But he noted even that force is enough to make it hard to move or concentrate, and that it can cause your cheeks to get "sucked down" while you "start seeing red dots" (because it draws blood away from the brain).

Musk recounted that "the most fun flight" he ever did was in an L-39 near Reno. It was a tail chase in which he followed another jet at a low altitude over the mountains.

"Literally, it was just like in Top Gun," he said. "You're no more than a couple of hundred feet above the ground, following the contour of the mountains."

Why Musk stopped flying military jets

Despite the thrill of such flights, Musk eventually stopped flying L-39s.

"I was like, man, this was made by some Soviet technician and maybe they tightened the bolt right, or maybe they didn't," Musk said, according to "The Space Barons" book. "Not a lot of redundancy. It was like, 'This is crazy. I've got kids. I have to stop doing this.'"

It's unclear when Musk dropped his high-risk hobby, but it happened more than a decade ago. His entrepreneurial successes and challenges had caught up with him by then, so he swapped his love of flying for designing rocket ships and electric cars.

"Sadly, I don't pilot myself any more," Musk wrote in September 2008 during a live web chat with the Washington Post. "I have to work when I fly and have too many thoughts in my head to pay the necessary attention to the plane — I can be absentminded at times, which is a really bad habit for a pilot."

Musk's passion for high-speed aviation is alive and well, though.

An illustration of SpaceX's "Big Falcon Rocket" flying passengers to another location on Earth.SpaceX/YouTube

"We're going to fly BFR like an aircraft and do point-to-point travel on Earth, so you can take off from New York City or Vancouver and fly halfway across the globe," Gwynne Shotwell, SpaceX's president and chief operating officer, said in April at a TED conference. "You'll be on the BFR for roughly half an hour, 40 minutes, and the longest part of that flight is actually the boat out [to the floating launch pad] and back."